In recent years, radio communication terminals such as PHSs (Personal Handy phone System), portable telephones, and the like have been widely available, and it has been possible to make calls or obtain information anywhere and anytime. In particular, lately, as an amount of available information has shown steady growth, high speed and high quality radio communication systems have been introduced to download a large amount of data.
As one of the high-speed digital radio communication systems, there is an OFDMA (Orthogonal Frequency Division Multiplex Access) system represented by IEEE802.11 or WiMAX (registered trademark) (e.g., Non-Patent Document 1). The OFDMA system is one of data multiplexing systems, which effectively uses a frequency band by using a plurality of carriers on a unit time axis and making phases of signal waves orthogonal between adjacent carriers to partially overlap bands of the carriers. In addition, compared to an OFDM (Orthogonal Frequency Division Multiplexing) system, which assigns sub-channels in time division to individual users, the OFDMA system allows a plurality of users to share all sub-channels and assigns sub-channels having the highest transmission efficiency to each user.
If the above-described next-generation high-speed radio communication method, such as WiMAX, uses high frequency range of 2.5 GHz or higher, communication coverage for one base station is reduced in an area where the radio wave state is poor due to many obstacles or other causes. Thus, in order to cover all environments, a plurality of base stations need to be installed. In addition, since detouring of radio waves is suppressed, it is highly likely that the inside of a building is out of the communication coverage.
Accordingly, the technology for reducing a range of a service area, where communication with a base station is not available due to an effect of shadowing, which occurs from hiding by obstacles such as a building, or other causes, by installing a relay device for relaying the base station and a radio terminal device is disclosed (e.g., Patent Document 1)